Derivative Accounting - Derivative Instruments in the Balance Sheet (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Assets
Gross Recognized Derivatives	$ 515,000	$ 1,113,000
Liabilities
Amount Reported on Balance Sheet	(72,132,000)	(60,037,000)
Commodity Contracts
Assets
Gross Recognized Derivatives		3,142,000
Amounts Offset		(2,185,000)
Net Recognized Derivatives		957,000
Other	405,000	156,000
Amount Reported on Balance Sheet		1,113,000
Liabilities
Gross Recognized Derivatives	(71,421,000)	(60,912,000)
Amounts Offset	474,000	2,185,000
Net Recognized Derivatives	(70,947,000)	(58,727,000)
Other	(1,185,000)	(1,310,000)
Amount Reported on Balance Sheet	(72,132,000)	(60,037,000)
Assets and Liabilities
Gross Recognized Derivatives	(70,837,000)	(57,770,000)
Amounts Offset	0	0
Net Recognized Derivatives	(70,837,000)	(57,770,000)
Other	(780,000)	(1,154,000)
Amount Reported on Balance Sheet	(71,617,000)	(58,924,000)
Commodity Contracts | Current Assets
Assets
Gross Recognized Derivatives	584,000	3,106,000
Amounts Offset	(474,000)	(2,149,000)
Net Recognized Derivatives	110,000	957,000
Other	405,000	156,000
Amount Reported on Balance Sheet	515,000	1,113,000
Commodity Contracts | Investments and Other Assets
Assets
Gross Recognized Derivatives		36,000
Amounts Offset		(36,000)
Net Recognized Derivatives		0
Other		0
Amount Reported on Balance Sheet		0
Commodity Contracts | Current Liabilities
Liabilities
Gross Recognized Derivatives	(38,235,000)	(36,345,000)
Amounts Offset	474,000	2,149,000
Net Recognized Derivatives	(37,761,000)	(34,196,000)
Other	(1,185,000)	(1,310,000)
Amount Reported on Balance Sheet	(38,946,000)	(35,506,000)
Commodity Contracts | Deferred Credits and Other
Liabilities
Gross Recognized Derivatives	(33,186,000)	(24,567,000)
Amounts Offset	0	36,000
Net Recognized Derivatives	(33,186,000)	(24,531,000)
Other	0	0
Amount Reported on Balance Sheet	$ (33,186,000)	$ (24,531,000)